Secondary battery, graphene oxide, and manufacturing method thereof

ABSTRACT

To provide a manufacturing method of graphene oxide that allows mass production through a relatively simple process, at low costs, and with safety and efficiency. A hydrogen peroxide solution, sulfuric acid, and flake graphite are put in a reaction container, and the mixture is stirred to obtain expansion graphite. The synthesized expansion graphite is washed not with pure water but with a saturated aqueous solution of magnesium sulfate (MgSO 4 ) or an organic solvent, whereby a large amount of sulfuric acid is contained between graphite layers. The expansion graphite is subjected to heat treatment or microwave irradiation to form expanded graphite, and a graphite layer is peeled by ultrasonic treatment and then oxidized to form a graphene compound.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a manufacturingmethod of graphene oxide. Another embodiment of the present inventionrelates to a secondary battery including graphene oxide and amanufacturing method thereof. Another embodiment of the presentinvention relates to a semiconductor device including a secondarybattery.

Note that in this specification, a semiconductor device refers to anydevice that can function by utilizing semiconductor characteristics. Anelectro-optical device, a semiconductor circuit, and an electronicdevice are all semiconductor devices.

2. Description of the Related Art

With the downsizing of semiconductor devices, materials with highelectrical conductivity and thermal conductivity have been required.Examples of the materials include graphene and graphene oxide. Recentresearch has been actively conducted on the use of carbon-basedmaterials such as graphene and graphene oxide for components ofbatteries (lithium-ion secondary batteries and capacitors).

Graphene is a material having high strength, electrical conductivity,thermal conductivity, and heat resistance. Graphene is a single atomiclayer consisting of six-membered carbon rings. Graphite is stackedlayers of graphene.

Graphene oxide is obtained by Hummers' method (graphite is oxidizedthrough the addition of concentrated sulfuric acid, sodium nitrate, andpotassium permanganate), or modified Hummers' method (through theaddition of KMnO₄ to a mixture of concentrated sulfuric acid, phosphoricacid, and graphite). Commercially available graphene oxide, which isfabricated not by Hummers' method producing toxic gases but by modifiedHummers' method or the like, is highly expensive.

Patent Document 1 discloses a method to obtain expansion graphite.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2012-131691

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a manufacturing methodof graphene oxide that allows mass production through a relativelysimple process, at low costs, and with safety and efficiency.

Another embodiment of the present invention provides an application ofgraphene oxide to part of a secondary battery.

Graphene oxide can be obtained at low costs by using graphite orexpansion graphite as a raw material and performing heating in anoxygen-containing atmosphere.

Expansion graphite refers to graphite intercalation compounds (GTC)where intercalant such as sulfuric acid is inserted between graphitelayers. In this specification, a substance resulting from gas releasefrom expansion graphite by heat treatment, microwave irradiation, or thelike is referred to as expanded graphite.

Note that it is difficult to peel a thin graphite layer from graphite orexpansion graphite. For example, a thin graphite layer cannot be easilyobtained from commercially available expansion graphite, which containsa small amount of sulfuric acid and thus a small number of graphitelayers where sulfuric acid is inserted. This causes a resulting graphitelayer to have a large thickness.

In addition, when synthesized expansion graphite is washed with purewater so that sulfuric acid is washed away, sulfuric acid interposedbetween graphite layers is released at the same time.

A structure of a manufacturing method of the present invention is amethod for manufacturing expansion graphite by the steps of insertingsulfuric acid between graphite layers and performing washing with anaqueous solution containing sulfate or an organic solvent.

For example, a hydrogen peroxide solution, sulfuric acid, and flakegraphite are put in a reaction container, and the mixture is stirred toobtain expansion graphite. The synthesized expansion graphite is washednot with pure water but with a saturated aqueous solution of magnesiumsulfate (MgSO₄) or a specific organic solvent. Thus, a large amount ofsulfuric acid can be contained between graphite layers. Instead ofmagnesium sulfate, sulfate such as potassium sulfate (K₂SO₄) or titaniumsulfate (Ti(SO₄)₂) may be used. An example of the specific organicsolvent is γ-butyrolactone.

The above structure can be a novel method for manufacturing expansiongraphite. When water is dropped into sulfuric acid, the water is boiledso that sulfuric acid is scattered from the dropping point and theperiphery thereof. This phenomenon does not occur in the case where asaturated aqueous solution of magnesium sulfate or a specific organicsolvent is used, which is advantageous in that a manufacturer performsthe process safely.

A novel method for manufacturing a graphene compound can be obtained inthe following manner. The expansion graphite that has been obtained bythe novel manufacturing method of expansion graphite is subjected toheat treatment or microwave irradiation to form expanded graphite, and agraphite layer is peeled by ultrasonic treatment and then oxidized toform a graphene compound.

Another novel method for manufacturing a graphene compound can also beobtained in the following manner. The expansion graphite that has beenobtained by the novel manufacturing method of expansion graphite issubjected to heat treatment or microwave irradiation to form expandedgraphite, and then oxidized and graphene oxide is formed by ultrasonictreatment.

The peeling is not necessarily performed by ultrasonic treatment and maybe performed in any step of applying a peeling force to expandedgraphite, such as a heating step or a mechanical peeling step.

The aforementioned ultrasonic treatment preferably includes at least astep of performing ultrasonic treatment at greater than or equal to 25kHz and less than or equal to 40 kHz, in which case a thin graphitelayer can be peeled. After the ultrasonic treatment, drying may beperformed by mixture and dispersion treatment using a highly dispersivedevice such as a bead mill, a roll mill, or FILMIX (registeredtrademark).

The manufacturing methods of expansion graphite and a graphene compounddisclosed in this specification, which have simple processes, consumelow energy and are produced at low costs and with low environmentalload, i.e., suitable for industrial production.

Note that in one embodiment of the present invention, a graphenecompound can be used in a component of a power storage device. Asdescribed later, when modification is performed, the structure andcharacteristics of a graphene compound can be selected from a widerrange of alternatives. Thus, a preferable property can be exhibited inaccordance with a component in which a graphene compound is to be used.Moreover, a graphene compound has a high mechanical strength andtherefore can be used in a component of a flexible power storage device.

A graphene compound obtained by any of the above novel manufacturingmethods of a graphene compound is mixed with an active material to forma paste, the paste is applied on a current collector to form a firstelectrode, and the first electrode overlaps with a second electrode witha separator positioned therebetween, whereby a secondary battery can beformed.

Another structure of the manufacturing method of the present inventionis a method for manufacturing a secondary battery by the steps ofinserting sulfuric acid between graphite layers, performing washing withan aqueous solution containing sulfate or an organic solvent, performingheating to form expanded graphite, performing ultrasonic treatment toform a peeled graphite layer, performing oxidation before or after theultrasonic treatment to form a graphene compound, mixing the oxidizedgraphene compound with an active material to form a paste, applying thepaste on a current collector to form a first electrode, and overlappingthe first electrode and a second electrode with a separator positionedtherebetween.

Graphene compounds will be described below.

Graphene has carbon atoms arranged in one atomic layer. A π bond existsbetween the carbon atoms. Graphene including two or more and one hundredor less layers is referred to as multilayer graphene in some cases. Thelength in the longitudinal direction or the length of the major axis ina plane in each of graphene and multilayer graphene is greater than orequal to 50 nm and less than or equal to 100 μm or greater than or equalto 800 nm and less than or equal to 50 μm.

In this specification and the like, a compound including graphene ormultilayer graphene as a basic skeleton is referred to as a graphenecompound. Graphene compounds include graphene and multilayer graphene.

Graphene compounds are detailed below.

A graphene compound is a compound where graphene or multilayer grapheneis modified with an atom other than carbon or an atomic group with anatom other than carbon. A graphene compound may be a compound wheregraphene or multilayer graphene is modified with an atomic groupcomposed mainly of carbon, such as an alkyl group or an alkylene group.An atomic group that modifies graphene or multilayer graphene isreferred to as a substituent, a functional group, a characteristicgroup, or the like in some cases. Modification in this specification andthe like refers to introduction of an atom other than carbon, an atomicgroup with an atom other than carbon, or an atomic group composed mainlyof carbon to graphene, multilayer graphene, a graphene compound, orgraphene oxide (described later) by a substitution reaction, an additionreaction, or other reactions.

Note that the front surface and the back surface of graphene may bemodified with different atoms or atomic groups. In multilayer graphene,multiple layers may be modified with different atoms or atomic groups.

An example of the above-described graphene modified with an atom or anatomic group is graphene or multilayer graphene that is modified withoxygen or a functional group containing oxygen. Examples of thefunctional group containing oxygen include an epoxy group, a carbonylgroup such as a carboxyl group, and a hydroxyl group. A graphenecompound modified with oxygen or a functional group containing oxygen isreferred to as graphene oxide in some cases. In this specification,graphene oxides include multilayer graphene oxides.

A formation method example of graphene oxide is described below.Graphene oxide can be obtained by oxidizing the aforementioned grapheneor multilayer graphene. Alternatively, graphene oxide can be obtained bybeing separated from graphite oxide. Graphite oxide can be obtained byoxidizing graphite. The graphene oxide may be further modified with theabove-mentioned atom or atomic group.

A compound that can be obtained by reducing graphene oxide is referredto as reduced graphene oxide (RGO) in some cases. In RGO, in some cases,all oxygen atoms contained in the graphene oxide are not extracted andpart of them remains in a state of oxygen or an atomic group containingoxygen that is bonded to carbon. In some cases, RGO includes afunctional group, e.g., an epoxy group, a carbonyl group such as acarboxyl group, or a hydroxyl group.

A graphene compound may have a sheet-like shape where a plurality ofgraphene compounds partly overlap each other. Such a graphene compoundis referred to as a graphene compound sheet in some cases. The graphenecompound sheet has, for example, an area with a thickness larger than orequal to 0.33 nm and smaller than or equal to 10 mm, preferably largerthan 0.34 nm and smaller than or equal to 10 μm. The graphene compoundsheet may be modified with an atom other than carbon, an atomic groupcontaining an atom other than carbon, an atomic group composed mainly ofcarbon such as an alkyl group, or the like. A plurality of layers in thegraphene compound sheet may be modified with different atoms or atomicgroups.

A graphene compound may have a five-membered ring composed of carbonatoms or a poly-membered ring that is a seven- or more-membered ringcomposed of carbon atoms, in addition to a six-membered ring composed ofcarbon atoms. In the neighborhood of a poly-membered ring which is aseven- or more-membered ring, a region through which a lithium ion canpass may be generated.

A plurality of graphene compounds may be gathered to form a sheet-likeshape

A graphene compound has a planar shape, thereby enabling surfacecontact.

In some cases, a graphene compound has high conductivity even when it isthin. The contact area between graphene compounds or between a graphenecompound and an active material can be increased by surface contact.Thus, even with a small amount of a graphene compound per volume, aconductive path can be formed efficiently.

In contrast, a graphene compound may also be used as an insulator. Forexample, a graphene compound sheet can be used as a sheet-likeinsulator. Graphene oxide, for example, has a higher insulation propertythan a graphene compound that is not oxidized, in some cases. A graphenecompound modified with an atomic group may have an improved insulationproperty, depending on the type of the modifying atomic group.

A graphene compound in this specification and the like may include aprecursor of graphene. The precursor of graphene refers to a substanceused for forming graphene. The precursor of graphene may contain theabove-described graphene oxide, graphite oxide, or the like.

Graphene containing an alkali metal or graphene containing an elementother than carbon, such as oxygen, is referred to as a graphene analogin some cases. In this specification and the like, graphene compoundsinclude graphene analogs.

A graphene compound in this specification and the like may include anatom, an atomic group, and ions of them between the layers. The physicalproperties, such as electric conductivity and ion conductivity, of agraphene compound sometimes change when an atom, an atomic group, andions of them exist between layers of the compound. In addition, adistance between the layers is increased in some cases.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. A modified graphene compound canhave an extremely low conductivity and serve as an insulator dependingon the type of the modification. A graphene compound has a planar shapeA graphene compound enables low-resistance surface contact.

A large amount of graphene oxide can be manufactured safely fromgraphite or expansion graphite. In addition, the manufacturing costs canbe reduced so that secondary batteries using graphene oxide come intowidespread use. Furthermore, the reduced manufacturing costs contributeto the widespread use of electronic devices including secondarybatteries whose positive electrodes use graphene oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 shows an example of a manufacturing flow of one embodiment of thepresent invention;

FIGS. 2A to 2C illustrate a coin-type storage battery;

FIGS. 3A and 3B illustrate a cylindrical storage battery;

FIGS. 4A and 4B illustrate a laminated storage battery;

FIGS. 5A and 5B illustrate application modes of a power storage device;

FIG. 6 is a graph showing XRD measurement results;

FIG. 7 is a graph showing XRD measurement results;

FIG. 8 is a cross-sectional SEM photograph of expansion graphite; and

FIG. 9 is a graph showing XRD measurement results.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that modes and details ofthe present invention can be modified in various ways. In addition, thepresent invention should not be construed as being limited to thedescription in the embodiments given below.

Embodiment 1

FIG. 1 shows a manufacturing flow example. Prepared first is a mixedsolution in which a hydrogen peroxide solution (H₂O₂) and concentratedsulfuric acid are sequentially added and mixed in a container (S101,S102).

Then, as a raw material powder, a powder of natural graphite or a powderof artificial graphite is prepared.

Subsequently, the natural or artificial graphite powder is added to themixed solution in which the hydrogen peroxide solution and concentratedsulfuric acid have been sequentially mixed in the container (S103). Thereaction among the graphite powder, the hydrogen peroxide solution, andconcentrated sulfuric acid may be performed in an air atmosphere or aninert gas atmosphere of argon, nitrogen, or the like. The reaction timeamong the graphite powder, the hydrogen peroxide solution, andconcentrated sulfuric acid is longer than or equal to 30 minutes andshorter than or equal to 48 hours, and the reaction temperature ishigher than or equal to 0° C. and lower than or equal to 50° C. A lowconcentration of the hydrogen peroxide solution causes sulfuric acidbetween graphite layers to flow out, thereby preventing a desired amountof sulfuric acid from being maintained. Thus, the concentration ofhydrogen peroxide in the hydrogen peroxide solution is set to greaterthan or equal to 20 wt % and less than or equal to 40 wt %. The reactionbetween the graphite powder and the hydrogen peroxide solution enables ahydroxyl group to be introduced into graphite. Then, expansion graphitein which sulfuric acid (or a sulfuric acid ion) is inserted betweengraphite layers is produced.

Next, washing is performed using an aqueous solution containing sulfate(S104), and then, drying is performed. The use of the aqueous solutioncontaining sulfate is preferable because sulfuric acid between graphitelayers can be prevented from flowing out Examples of the aqueoussolution containing sulfate include an aqueous solution of magnesiumsulfate, an aqueous solution of potassium sulfate, and an aqueoussolution of titanium sulfate. More preferably, a saturated aqueoussolution of sulfuric acid is used in order to further prevent the flowout of sulfuric acid. For example, a saturated aqueous solution ofmagnesium sulfate, a saturated aqueous solution of potassium sulfate, ora saturated aqueous solution of titanium sulfate can be used. An organicsolvent may be used instead of the aqueous solution containing sulfate.The use of the organic solvent prevents the reduction of graphite thathas been oxidized in washing. As the organic solvent, for example,γ-butyrolactone can be used.

Subsequently, heat treatment is performed in an air atmosphere at higherthan or equal to 120° C. and lower than or equal to 1050° C. for longerthan or equal to 1 minute and shorter than or equal to 24 hours (S105).The heat treatment at higher than or equal to 120° C. gasifies sulfuricacid in graphite and makes a space between the graphite layers. Thespace expands to produce expanded graphite. The heat treatment forobtaining expanded graphite may be microwave irradiation with amicrowave oven or the like.

Then, oxidation treatment is performed by heat treatment in an oxygenatmosphere (S106). The heat treatment in an oxygen atmosphere isperformed at higher than or equal to 150° C. and lower than or equal to1000° C. for longer than or equal to 30 minutes and shorter than orequal to 24 hours.

Next, ultrasonic treatment for peeling is performed (S107). Mechanicalpeeling treatment may be performed instead of the ultrasonic treatment.

The ultrasonic treatment is performed at greater than or equal to 25 kHzand less than or equal to 40 kHz in a dispersion medium such as ethanolor N-methylpyrrolidone (NMP). The irradiation time of ultrasonictreatment in the dispersion medium is, but not particularly limited to,longer than or equal to 5 minutes and shorter than or equal to 2 hours.

Note that for easy dispersion in the dispersion medium, graphite may bemodified with a functional group at low costs.

In the case where mechanical peeling treatment is performed, FILMIX(registered trademark) may be employed. Ultrasonic treatment andmechanical peeling treatment may be alternately performed for peeling.

Peeling may be performed by filtration, extraction, washing, drying,separation and purification such as centrifugation, or a combination ofany of them.

In the case where the peeled thin graphite layer needs to be oxidized,heat treatment may be further performed in an oxygen atmosphere. Throughthe above steps, graphene oxide can be manufactured at low costs.

The use of such graphene oxide for a positive electrode material or thelike reduces the total production costs of secondary batteries

Embodiment 2

In this embodiment, the structure of a storage battery includinggraphene oxide formed by the method described in Embodiment 1 as aconductive additive will be described with reference to FIGS. 2A to 4B.

FIG. 2A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 2B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. The positiveelectrode active material layer 306 may further include a binder forincreasing the adhesion of positive electrode active materials, aconductive additive for increasing the conductivity of the positiveelectrode active material layer, and the like in addition to the activematerials. As the conductive additive, a material that has a largespecific surface area is preferably used; for example, acetylene black(AB) can be used. Alternatively, a carbon material such as a carbonnanotube, graphene, or fullerene can be used.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308. The negativeelectrode active material layer 309 may further include a binder forincreasing the adhesion of negative electrode active materials, aconductive additive for increasing the conductivity of the negativeelectrode active material layer, and the like in addition to thenegative electrode active materials. A separator 310 and an electrolyte(not illustrated) are provided between the positive electrode activematerial layer 306 and the negative electrode active material layer 309.

A material with which lithium can be dissolved and precipitated or amaterial into and from which lithium ions can be inserted and extractedcan be used for the negative electrode active materials used for thenegative electrode active material layer 309; for example, a lithiummetal, a carbon-based material, and an alloy-based material can be used.The lithium metal is preferable because of its low redox potential(3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, and carbon black.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (e.g., 0.1 V to 0.3 V vs. Li/Li⁺) when lithium ions areintercalated into the graphite (while a lithium-graphite intercalationcompound is formed). For this reason, a lithium-ion secondary batterycan have a high operating voltage. In addition, graphite is preferablebecause of its advantages such as relatively high capacity per unitvolume, small volume expansion, low costs, and safety greater than thatof a lithium metal.

For the negative electrode active materials, an alloy-based materialthat enables charge-discharge reactions by an alloying reaction and adealloying reaction with lithium metal can be used. In the case wherecarrier ions are lithium ions, a material containing at least one of Ga,Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Au, Zn, Cd, In, and the like can be usedfor example. Such elements have higher capacity than carbon. Inparticular, silicon has a significantly high theoretical capacity of4200 mAh/g. For this reason, silicon is preferably used for the negativeelectrode active materials. Examples of the alloy-based material usingsuch elements include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂, Mg₂Sn, SnS₂, V₂Sn₃,FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃,La₃Co₂Sn₇, CoSb₃, InSb, and SbSn. Here, SiO refers to a material thatcontains silicon at higher proportion than SiO₂ does.

Alternatively, for the negative electrode active materials, an oxidesuch as titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active materials,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive materials and thus the negative electrode active materials can beused in combination with a positive electrode active material that doesnot contain lithium ions, such as V₂O₅ or Cr₃O₈. In the case of using amaterial containing lithium ions as a positive electrode activematerial, the nitride containing lithium and a transition metal can beused for the negative electrode active material by extracting thelithium ions contained in the positive electrode active material inadvance.

Alternatively, a material that causes a conversion reaction can be usedfor the negative electrode active materials; for example, a transitionmetal oxide which does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may beused. Other examples of the material that causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

The current collectors 305 and 308 can each be formed using a highlyconductive material which is not alloyed with a carrier ion of, forexample, lithium, such as a metal typified by stainless steel, gold,platinum, zinc, iron, nickel, copper, aluminum, titanium, and tantalumor an alloy thereof. Alternatively, an aluminum alloy to which anelement that improves heat resistance, such as silicon, titanium,neodymium, scandium, or molybdenum, is added can be used. Stillalternatively, a metal element that forms silicide by reacting withsilicon can be used. Examples of the metal element that forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel.The current collectors can each have a foil-like shape, a plate-likeshape (sheet-like shape), a net-like shape, a cylindrical shape, a coilshape, a punching-metal shape, an expanded-metal shape, or the like asappropriate. The current collectors each preferably have a thicknessgreater than or equal to 5 μm and less than or equal to 30 μm.

The positive electrode active materials described in Embodiment 1 can beused for the positive electrode active material layer 306.

The separator 310 can be formed using an insulator such as cellulose(paper), polyethylene with pores, or polypropylene with pores.

For an electrolyte in an electrolyte solution, a material containingcarrier ions is used. Typical examples of the electrolyte are lithiumsalts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, andLi(C₂FsSO₂)₂N. One of these electrolytes may be used alone, or two ormore of them may be used in an appropriate combination and in anappropriate ratio.

Note that when carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, instead of lithium in the abovelithium salts, an alkali metal (e.g., sodium and potassium), analkaline-earth metal (e.g., calcium, strontium, barium, beryllium, andmagnesium) may be used for the electrolyte.

For a solvent of the electrolyte solution, a material having the carrierion mobility is used. As the solvent of the electrolyte solution, anaprotic organic solvent is preferably used. Typical examples of aproticorganic solvents include ethylene carbonate (EC), propylene carbonate,dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,dimethoxyethane, and tetrahydrofuran, and one or more of these materialscan be used. When a gelled polymeric material is used for the solvent ofthe electrolyte solution, safety against liquid leakage and the like isimproved. Furthermore, a thin and light storage battery can befabricated. Typical examples of gelled polymeric materials include asilicone gel, an acrylic gel, an acrylonitrile gel, a polyethyleneoxide-based gel, a polypropylene oxide-based gel, and a gel of afluorine-based polymer. Alternatively, the use of one or more kinds ofionic liquids (room temperature molten salts) which have features ofnon-flammability and non-volatility for the solvent of the electrolytesolution can prevent the storage battery from exploding or catching fireeven when the storage battery internally shorts out or the internaltemperature increases because of overcharging and the like.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including ahigh-molecular material such as a polyethylene oxide (PEO)-basedhigh-molecular material may be used. When the solid electrolyte is used,a separator and a spacer are not necessary. Furthermore, the battery canbe entirely solidified; therefore, there is no possibility of liquidleakage and thus the safety of the battery is dramatically increased.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolyte solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel) can beused. Alternatively, the positive electrode can 301 and the negativeelectrode can 302 are preferably covered with nickel, aluminum, or thelike in order to prevent corrosion due to the electrolyte solution. Thepositive electrode can 301 and the negative electrode can 302 areelectrically connected to the positive electrode 304 and the negativeelectrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte solution. Then, asillustrated in FIG. 2B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type storage battery300 can be manufactured.

Here, a current flow in charging a battery is described with referenceto FIG. 2C. When a battery using lithium is regarded as a closedcircuit, lithium ions transfer and a current flows in the samedirection. Note that in the battery using lithium, an anode and acathode change places in charge and discharge, and an oxidation reactionand a reduction reaction occur on the corresponding sides; hence, anelectrode with a high redox potential is called a positive electrode andan electrode with a low redox potential is called a negative electrode.For this reason, in this specification, the positive electrode isreferred to as a “positive electrode” or a “plus electrode” and thenegative electrode is referred to as a “negative electrode” or a “minuselectrode” in all the cases where charge is performed, discharge isperformed, a reverse pulse current is supplied, and a charging currentis supplied. The use of the terms “anode” and “cathode” related to anoxidation reaction and a reduction reaction might cause confusionbecause the anode and the cathode change places at the time of chargingand discharging. Thus, the terms “anode” and “cathode” are not used inthis specification. If the term “anode” or “cathode” is used, it shouldbe mentioned that the anode or the cathode is which of the one at thetime of charging or the one at the time of discharging and correspondsto which of a positive (plus) electrode or a negative (minus) electrode.

Two terminals in FIG. 2C are connected to a charger, and a storagebattery 400 is charged. As the charge of the storage battery 400proceeds, a potential difference between electrodes increases. In FIG.2C, the positive direction is the direction in which a current flowsfrom a terminal outside the storage battery 400 to a positive electrode402, flows from the positive electrode 402 to a negative electrode 404in the storage battery 400, and flows from the negative electrode to theother terminal outside the storage battery 400. In other words, adirection in which a charging current flows is regarded as a directionof a current.

[Cylindrical Storage Battery]

Next, an example of a cylindrical storage battery will be described withreference to FIGS. 3A and 3B. A cylindrical storage battery 600includes, as illustrated in FIG. 3A, a positive electrode cap (batterylid) 601 on the top surface and a battery can (outer can) 602 on theside and bottom surfaces. The positive electrode cap and the battery can(outer can) 602 are insulated from each other by a gasket (insulatinggasket) 610.

FIG. 3B is a schematic cross-sectional view of the cylindrical storagebattery. Inside the battery can 602 having a hollow cylindrical shape, abattery element in which a strip-like positive electrode 604 and astrip-like negative electrode 606 are wound with a strip-like separator605 interposed therebetween is provided. Although not illustrated, thebattery element is wound around a center pin. One end of the battery can602 is close and the other end thereof is open. For the battery can 602,a metal having a corrosion-resistant property to an electrolytesolution, such as nickel, aluminum, or titanium, an alloy of such ametal, or an alloy of such a metal and another metal (e.g., stainlesssteel) can be used. Alternatively, the battery can 602 is preferablycovered with nickel, aluminum, or the like in order to prevent corrosiondue to the electrolyte solution. Inside the battery can 602, the batteryelement in which the positive electrode, the negative electrode, and theseparator are wound is provided between a pair of insulating plates 608and 609 that face each other. Furthermore, a nonaqueous electrolytesolution (not illustrated) is injected inside the battery can 602provided with the battery element. As the nonaqueous electrolytesolution, a nonaqueous electrolyte solution that is similar to that ofthe coin-type storage battery can be used.

Although the positive electrode 604 and the negative electrode 606 canbe formed in a manner similar to that of the positive electrode and thenegative electrode of the coin-type storage battery described above, thedifference lies in that, since the positive electrode and the negativeelectrode of the cylindrical storage battery are wound, active materialsare formed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. The PTC element 611, which serves as a thermally sensitiveresistor whose resistance increases as temperature rises, limits theamount of current by increasing the resistance, in order to preventabnormal heat generation. Note that barium titanate (BaTiO₃)-basedsemiconductor ceramic can be used for the PTC element.

[Laminated Storage Battery]

Next, an example of a laminated storage battery will be described withreference to FIG. 4A. When the laminated storage battery has flexibilityand is used in an electronic device at least part of which is flexible,the storage battery can be bent as the electronic device is bent.

A laminated storage battery 500 illustrated in FIG. 4A includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolyte solution 508, and an exterior body 509. The separator 507 isplaced between the positive electrode 503 and the negative electrode506, which are provided in the exterior body 509. The exterior body 509is filled with the electrolyte solution 508. The positive electrodeactive materials described in Embodiment 1 can be used for the positiveelectrode active material layer 502.

In the laminated storage battery 500 illustrated in FIG. 4A, thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also serve as terminals for an electrical contactwith an external portion. For this reason, the positive electrodecurrent collector 501 and the negative electrode current collector 504may be arranged so as to be partly exposed to the outside of theexterior body 509. Alternatively, a lead electrode and the positiveelectrode current collector 501 or the negative electrode currentcollector 504 may be bonded to each other by ultrasonic welding, andinstead of the positive electrode current collector 501 and the negativeelectrode current collector 504, the tab electrode may be exposed to theoutside of the exterior body 509.

As the exterior body 509 in the laminated storage battery 500, forexample, a laminate film having a three-layer structure can be employedin which a highly flexible metal thin film of aluminum, stainless steel,copper, nickel, or the like is provided over a film formed of a materialsuch as polyethylene, polypropylene, polycarbonate, ionomer, orpolyamide, and an insulating synthetic resin film of a polyamide-basedresin, a polyester-based resin, or the like is provided over the metalthin film as the outer surface of the exterior body.

FIG. 4B illustrates an example of a cross-sectional structure of thelaminated storage battery 500. Although FIG. 4A illustrates an exampleincluding only two current collectors for simplicity, an actual batteryincludes a plurality of electrode layers.

The example in FIG. 4B includes 16 electrode layers. The laminatedstorage battery 500 has flexibility even though including 16 electrodelayers. FIG. 4B illustrates a structure including 8 layers of negativeelectrode current collectors 504 and 8 layers of positive electrodecurrent collectors 501, i.e., 16 layers in total. Note that FIG. 4Billustrates a cross section of the lead portion of the negativeelectrode, and the 8 negative electrode current collectors 504 arebonded to each other by ultrasonic welding. It is needless to say thatthe number of electrode layers is not limited to 16, and may be morethan 16 or less than 16. With a large number of electrode layers, thestorage battery can have high capacity. In contrast, with a small numberof electrode layers, the storage battery can have small thickness andhigh flexibility.

Embodiment 3 [Examples of Electrical Devices; Vehicles]

Described next are examples of vehicle including storage batteries. Theuse of storage batteries in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 5A and 5B each illustrate an example of a vehicle fabricated usingone embodiment of the present invention. An automobile 8100 illustratedin FIG. 5A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8100 is a hybrid electric vehiclecapable of driving using either an electric motor or an engine asappropriate. One embodiment of the present invention offers a vehicleincluding an inexpensive storage battery. The automobile 8100 includes apower storage device. The power storage device is used not only to drivethe electric motor, but also to supply electric power to alight-emitting device such as a headlight 8101 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8100. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8100, such as a navigation system.

FIG. 5B illustrates an automobile 8200 including a power storage device.The automobile 8200 can be charged when the power storage device issupplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.5B, the power storage device included in the automobile 8200 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. With the use of a plug-in technique, thepower storage device included in the automobile 8200 can be charged bybeing supplied with electric power from outside. The charging can beperformed by converting AC electric power into DC electric power througha converter such as an ACDC converter.

Although not illustrated, the vehicle may include a power receivingdevice so that it can be charged by being supplied with electric powerfrom an above-ground power transmitting device in a contactless manner.In the case of the contactless power feeding system, by fitting a powertransmitting device in a road or an exterior wall, charging can beperformed not only when the electric vehicle stops but also when moves.In addition, the contactless power feeding system may be utilized toperform transmission and reception of electric power between vehicles. Asolar cell may be provided in the exterior of the automobile to chargethe power storage device when the automobile stops or moves. To supplyelectric power in such a contactless manner, an electromagneticinduction method or a magnetic resonance method can be used.

According to one embodiment of the present invention, the power storagedevice fabricated at low costs contributes to a reduction in the costsof the vehicle.

This embodiment can be implemented in appropriate combination with theother embodiments.

Example 1

In this example, an example of a manufacturing method of expansiongraphite will be described below.

To 1 ml of hydrogen peroxide solution (H₂O₂) with 31 wt %, 22.5 ml ofconcentrated sulfuric acid (96%) is gradually added and the mixture isstirred. To the mixture, 5 g of graphite powder is added and stirred for1 hour at room temperature. In this example, flake graphite (F #1manufactured by Nippon Graphite Industries, Co., Ltd.) with an averageparticle size of 500 μm is used as the graphite powder.

Then, concentrated sulfuric acid is removed as much as possible with asuction filtration apparatus, and washing is performed with a saturatedaqueous solution of magnesium sulfate. Washing is performed not withpure water but with an aqueous solution containing a large amount ofsulfuric acid ion, thereby preventing the release of sulfuric acid fromgraphite. Furthermore, sulfate prevents heat generation due to themixture of water and concentrated sulfuric acid.

FIG. 6 shows the XRD measurement results of graphite before washing. Asshown in FIG. 6, first-stage expansion graphite is obtained and the XRDspectrum suggests that the interlayer distance is approximately 8.02angstroms. FIG. 7 shows the XRD measurement results of graphite that hasbeen washed with a saturated aqueous solution of magnesium sulfate. Asshown in FIG. 7, seventh-stage expansion graphite is obtained. The stagenumber indicates how many atoms and molecules are intercalated into agraphite interlayer compound. For example, the second-stage graphiterefers to a substance in which sulfuric acid (or a sulfuric acid ion) isinserted in every two graphite layers. Hence, from graphite with a lowerstage, a thinner graphite layer can be peeled after thermal expansion.

The obtained expansion graphite is dried at 80° C.

Subsequently, heat treatment is performed in an air atmosphere at 600°C. for longer than or equal to 1 hour and shorter than or equal to 2hours, whereby expanded graphite is obtained. The heat treatment forobtaining expanded graphite may be microwave irradiation with amicrowave oven. With a microwave oven (600 W), 10-second treatment isrepeated twice to obtain expanded graphite.

After that, the expanded graphite is peeled by ultrasonic treatment at25 kHz in ethanol, and oxidized to manufacture graphene oxide. In thismanner, graphene oxide can be safely and efficiently mass-produced atlow costs.

Note that to observe the expanded graphite immediately after themicrowave irradiation with the microwave oven, 0.05 g of expandedgraphite is extracted and subjected to ultrasonic treatment at 25 kHz inethanol (10 mL) for approximately 10 minutes. FIG. 8 is a photograph ofthe SEM observation.

Example 2

In this example, an example of using an organic solvent instead of asaturated aqueous solution of magnesium sulfate will be described below.

In order to prevent the reduction of graphite that has been oxidized,γ-butyrolactone is used as an organic solvent in washing.

This example is different from Example 1 only in the liquid used inwashing; thus, the description is omitted.

FIG. 9 shows the XRD measurement results of graphite that has beenwashed. The XRD spectrum suggests that the interlayer distance isapproximately 8 angstroms. As shown in FIG. 9, third-stage orfourth-stage expansion graphite is obtained.

Subsequently, heat treatment is performed in an air atmosphere forlonger than or equal to 1 hour and shorter than or equal to 2 hours,whereby expanded graphite is obtained. The heat treatment for obtainingexpanded graphite may be microwave irradiation with a microwave oven.After that, the expanded graphite is peeled by ultrasonic treatment at25 kHz in ethanol (approximately 10 minutes), and oxidized tomanufacture graphene oxide. In this manner, graphene oxide can be safelyand efficiently mass-produced at low costs.

This application is based on Japanese Patent Application serial No.2015-250970 filed with Japan Patent Office on Dec. 24, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a graphene compound,comprising steps of: adding graphite into solution including sulfuricacid; washing the graphite with an aqueous solution containing sulfateor an organic solvent after adding; forming expanded graphite from thegraphite after washing; and performing ultrasonic treatment on theexpanded graphite.
 2. The method for manufacturing a graphene compoundaccording to claim 1, wherein, in the step of the ultrasonic treatment,the expanded graphite is dispersed in a dispersed medium.
 3. The methodfor manufacturing a graphene compound according to claim 2, wherein thedispersed medium is ethanol.
 4. The method for manufacturing a graphenecompound according to claim 1, wherein the expanded graphite is formedby heat treatment or microwave irradiation.
 5. The method formanufacturing a graphene compound according to claim 1, wherein thegraphite is flake graphite.
 6. The method for manufacturing a graphenecompound according to claim 1, wherein the sulfate is magnesium sulfate,potassium sulfate, or titanium sulfate.
 7. The method for manufacturinga graphene compound according to claim 1, wherein the aqueous solutioncontaining sulfate is a saturated aqueous solution of magnesium sulfate,a saturated aqueous solution of potassium sulfate, or a saturatedaqueous solution of titanium sulfate.
 8. The method for manufacturing agraphene compound according to claim 1, wherein the organic solvent isγ-butyrolactone.
 9. A method for manufacturing a secondary battery,comprising the steps of adding graphite into solution including sulfuricacid; washing the graphite with an aqueous solution containing sulfateor an organic solvent after adding; forming expanded graphite from thegraphite after washing; performing ultrasonic treatment on the expandedgraphite to form graphene oxide; mixing the graphene oxide with anactive material to form a paste; applying the paste on a currentcollector to form a first electrode; and overlapping the first electrodeand a second electrode with a separator positioned therebetween.
 10. Themethod for manufacturing a secondary battery according to claim 9,wherein, in the step of the ultrasonic treatment, the expanded graphiteis dispersed in a dispersed medium.
 11. The method for manufacturing asecondary battery according to claim 10, wherein the dispersed medium isethanol.
 12. The method for manufacturing a secondary battery accordingto claim 9, wherein the expanded graphite is formed by heat treatment ormicrowave irradiation.
 13. The method for manufacturing a secondarybattery according to claim 9, wherein the graphite is flake graphite.14. The method for manufacturing a secondary battery according to claim9, wherein the sulfate is magnesium sulfate, potassium sulfate, ortitanium sulfate.
 15. The method for manufacturing a secondary batteryaccording to claim 9, wherein the aqueous solution containing sulfate isa saturated aqueous solution of magnesium sulfate, a saturated aqueoussolution of potassium sulfate, or a saturated aqueous solution oftitanium sulfate.
 16. The method for manufacturing a secondary batteryaccording to claim 9, wherein the organic solvent is γ-butyrolactone.